The present invention relates to an exhaust apparatus for a process gas, which is used in combination with a process apparatus for forming a layer on an object to be processed using the process gas, and relates to a method of removing an impurity gas (unprocessed gas, non-reacted process gas) formed by a process gas.
In general, to form an integrated circuit, such as an IC, or a logic device, a step of depositing a desired thin film on the surface of an object to be processed, such as a semiconductor wafer, glass substrate or LCD substrate, and a step of etching the deposited film to a desired pattern are repeatedly carried out.
With regard to the film deposition step, for example, a thin silicon film, a thin film of silicon oxide or nitride, a thin metal film, a thin film of metal oxide or nitride, or the like is formed on a to-be-processed object by reacting a predetermined process gas (source gas) in a process vessel. It is known that an excess reaction by-product is produced at the same time as the reaction for film deposition and the reaction by-product, and a non-reacted process gas is discharged together with an exhaust gas.
The reaction by-product and non-reacted process gas, if discharged directly in an atmosphere, would cause an environmental pollution or the like. To prevent such pollution, an exhaust apparatus is connected to the process vessel. The exhaust apparatus has a trap mechanism provided in an exhaust gas system that extends from the process vessel to trap and removes a reaction by-product or non-reacted process gas or the like contained in the exhaust gas.
Various structures have been proposed for the trap mechanism in accordance with the characteristics of the reaction by-product or the like that should be trapped and removed. To eliminate a reaction by-product which is liquefied or solidified and condensed at an ordinary temperature, for example, a trap mechanism has multiple fins provided in a casing which has an inlet port and an exhaust port for the exhaust gas. The fins are orderly arranged in the flow direction of the exhaust gas and trap a reaction by-product or the like contained in the exhaust gas on their surfaces as the exhaust gas passes between the fins. Such an apparatus typically cools the fins with a cooling fluid or the like in order to improve the trapping efficiency.
A description will now be given of the case where a TiCl4 (titanium tetrachloride) gas of a high-melting point metal halogen compound is used as a source gas to form a Ti metal film on a semiconductor wafer. H2 gas is used as a source gas in addition to the TiCl4 gas. The H2 gas is activated by plasma in the process container under an Ar gas atmosphere and reduced with hydrogen, thus depositing a Ti film on the surface of the semiconductor wafer. At this time, TiClx (x<4) is produced as a reaction by-product, and a non-reacted TiCl4 gas is present in the process vessel. The TiClx gas and TiCl4 gas or the like flow out of the exhaust apparatus in the form of impurities in the exhaust gas. Because the TiClx gas and TiCl4 gas or the like are impurity gases that would cause air pollution or the like, they should be trapped by the aforementioned trap mechanism in the exhaust apparatus.
Because the aforementioned impurity gases, such as the TiCl4 gas or a non-reacted gas and the TiClx gas or a reaction by-product, have relatively high vapor pressures, it is very difficult to completely trap and eliminate those gases in the trap mechanism even if the interior of the trap mechanism is cooled as mentioned above. This may result in insufficient trapping. In this respect, an eliminator is provided at the downstream of the trap mechanism to completely eliminate the impurity gas that has passed through the trap mechanism. Such an eliminator is complicated and should be inspected frequently, leading to a higher running cost and a shorter service life. To solve this problem, the eliminator should have a very large capacity, which inevitably enlarges the whole apparatus and increases the cost. Such a shortcoming is common to various process apparatuses which use a high-melting point metal halogen compound gas such as TiCl4, WF6 or (Ta(OE)5)2 (pentoethoxy tantalum).
A method of depositing a TiN film is known as another process method which uses TiCl4 gas. This method will be explained with reference to the case where TiCl4 (titanium tetrachloride) gas of a high-melting point metal halogen compound is used as a source gas to form a TiN film. NH3 gas is used as a source gas in addition to the TiCl4 gas and both gases are reacted in a reactor to deposit a TiN film on the surface of a semiconductor wafer. At this time, NH4Cl and TiCl4(NH3)n (n: a positive integer) are produced as reaction by-products, and non-reacted TiCl4 gas is also present in the reactor. The gas components flow out of the reactor in the form of impurities in the exhaust gas and are trapped by the aforementioned trap mechanism and/or eliminator.
Because an unnecessary film which causes particles sticks on the inner wall of the process chamber of the process apparatus or the surface of a structure inside the vessel as the film deposition is carried out, cleaning is executed as needed, which regularly or irregularly supplies a cleaning gas into the process chamber to eliminate the unnecessary film. In this case, various kinds of fluorohalogen-based gases, such as ClF3 gas, are used as cleaning gases. The ClF3 gas removes the unnecessary film stuck on the inner wall or the like of the process chamber and is reacted with a reaction by-product of TiCl4(NH3)n, thus yielding another reaction by-product, such as TiF4(NH3)n.
As NH4Cl, TiCl4(NH3)n, TiF4(NH3)n, etc. are sequentially stored in the trap mechanism as reaction by-products, the trap mechanism is regularly or irregularly detached from a vacuum exhaust system, opening the interior so that the reaction by-products are cleaned out. At the time the trap mechanism is released in an atmosphere, NH4Cl hardly makes a problem because it is relatively stable. However, TiCl4(NH3)n or TiF4(NH3)n produce HCl gas, HF gas and NH3 gas, harmful to human bodies, as indicated by the following formulas (1), (2) if reacting oxygen in the air. Some countermeasures are therefore demanded.TiCl4(NH3)n+O2→TiO2+HCl+NH3  (1)TiF4(NH3)n+O2→TiO2+HF+NH2  (2)